Hanny's ghostly green blob

And then, the green blob stretched out to engulf the Milky Way! You could be forgiven for thinking that is what is going on in this image, which resembles a poster for a forgotten sci-fi flick.

In fact, it was taken by the Hubble Space Telescope and shows a stream of gas stretching around a nearby spiral galaxy. Only part of the stream is visible, illuminated by a light beam emitted by the quasar at the centre of the galaxy. The oxygen in the gas gives it the ghostly green glow.

Hanny's Voorwerp (Dutch for Hanny's object), as the stream has been nicknamed, is 300,000 light years long, the width of the Milky Way and is thought to have formed a billion years ago.

According to NASA, the "warped spiral arms" and "complex dust patches" in this image suggest that the gas stream has been disturbed, perhaps because of a galaxy merger.